1. Field of the Invention
A lubrication management system is disclosed for the operation of machines requiring a minimum level of lubricant in a lubricant sump. More particularly, a lubrication management system for refrigeration system compressors utilities a float member with a magnetically conductive portion in combination with a displacement transducer to provide an automatic feedback signal to a circuit for control of a normally closed solenoid valve in selective communication with a lubricant reservoir, such that the circuit can control the level of lubricant in the sump through replenishment from the reservoir to at least a minimun required level.
2. Description of the Prior Art
In commercial and industrial refrigeration systems, particularly units with parallel compressors, there is a need to accurately control the lubricant level in the compressor's sump or crankcase. Most refrigeration compressors utilize liquid lubricants for lubrication and internal cooling. The compressor is the heart of a refrigeration system and the most expensive component, typically costing more than all the other parts combined. If the proper lubricant level is not obtained and maintained (i.e., a lubricant sump level that is too high or too low), the compressor can be quickly destroyed through "burn-out" (the insufficient lubricant condition) or rendered grossly inefficient or "slugged" (the excessive lubricant condition).
Moreover, insufficient lubricant and resulting compressor "burn-out" can cause contamination of the refrigerant system and refrigerant plumbing. Cleaning out a refrigeration system contaminated with from a "burn-out" can often be one of the most costly service charges a unit will ever require other than replacement of the compressor. Indeed, all refrigeration compressors pass or "pump" lubricant through their refrigerant discharge lines due to machining tolerance limitations, the refrigerant fluid's miscibility with most lubricants, overall compressor design, pressure drops across the sump and various other design or system factors.
This lubricant contamination of the refrigerant discharge lines is even more significant with parallel compressor systems which utilize several refrigeration compressors, usually of a semi-hermetic type, sharing a common suction and discharge line. When lubricant is inadvertently pumped out of the compressor sump into the refrigerant circuit, the lubricant is directed through the system's evaporator, expansion valve, and condenser, and is eventually returned to the compressors still entrained in the refrigerant. Since such parallel compressor configurations share a common refrigerant supply or suction line and a common suction manifold at different locations while often operating under different duty cycles, the returning refrigerant (and lubricant contaminant) does not necessarily return to the exact compressor from whence it came at the same rate or quantity. In the presence of a common suction line and different duty cycles, lubricant can move from one compressor to another. Thus, minimization of lubricant contamination of the refrigerant and plumbing is a very high priority of virtually all systems.
A reverse contamination process is also possible, where the feed lubricant may contain entrained refrigerant. When lubricant is allowed into the compressor sump, it is possible that this contaminated refrigerant will circulate within the sump and be drawn into the compression chamber of the compressor. The common refrigerant supply or suction line and a common suction manifold can thus be further contaminated with undesirable lubricant.
One or more lubricant separators are commonly used on the compressor discharge lines to lower the amount of lubricant lost to the refrigerant. The lubricant is captured by the lubricant separator and returned to a common remote lubricant reservoir, previously filled with a large pre-charge of lubricant. The lubricant in the common lubricant reservoir is then ready to be returned to the individual compressor sumps as needed.
Over the years, there have developed several methods to maintain a minimum level of lubricants in the compressor sumps, as well as generally any fluid in a containment vessel. Such methods have been especially devoted to the needs of internal combustion engines and compressors, where hydraulic delivery of lubricants such as oil to the machine elements in relative motion is an absolute necessity in avoiding premature wear of the moving elements. Typically, such machines incorporate a sump or similar containment vessel placed in the lower regions of the machine for storage of the lubricant while a separate lubricant portion is circulating throughout the machine and delivered to the machine elements likely to experience wear. Thus, as a minimum level of lubricant in the sump is necessary to maintain the wear reducing and cooling function of the lubricant, it is often necessary to replenish the sump with additional lubricant during or after machine operation in the face of the lubricant losses caused by the reasons stated above. To properly accomplish this replenishment and lubricant management, it is first necessary to measure the existing level of the lubricant in the sump.
The most basic method of managing the level of lubricant in the sump as described above is through a dip stick or sight glass. With a dip stick, the level of oil is measured by the insertion of a stick of known and calibrated length into the lubricant sump a known and fixed distance. The level of the lubricant, adhering to the stick when the stick is withdrawn from the sump, can be readily determined by the length of adhered lubricant on the stick compared to the calibration marks on the stick. Such methods, although extremely commonplace, suffer from the disadvantage that only the oil existing in the sump at the time of the measurement can be determined. Subsequent loss of lubricant, such as through leaks, will not be determined until subsequent dip stick measurements are taken. Moreover, if it is determined that lubricant must be added, this addition must occur as a separate step and is often a messy and imprecise undertaking. In extreme cases, overfilling of the lubricant sump can create hydrolocking of the machine, where the pump hydraulically delivering the lubricant is caused to attempt to compress a column of lubricant, an essentially incompressible fluid. Severe structural damage is often caused during such events. Further, where multiple machines are in service, a common arrangement for stationary installations such as refrigeration system compressors, the need for more frequent attention increases.
Subsequent refinements include the use of lubricant pressure sensors in conjunction with the use of dip sticks. The pressure sensors measure the pressure generated during the hydraulic delivery of the lubricant to the wear elements, typically through comparative resistance sensors or piezo electric transducers. If the pressure drops below a predetermined level, the sensor indicates that the lubricant sump is in need of additional lubricant. However, such systems usually signal the need for additional lubricant only after an extreme need arises and are incapable themselves of adding the additional lubricant necessary.
Alternatives to dip stick methods include sight glasses provided in the side of the sump itself. Usually provided in stationary engines or compressors, visual inspection of the sight glass allows an operator to continuously monitor the level of lubricant in the sump and take appropriate action to add lubricant if the level falls below some predetermined level. However, such systems again only provide an indication of the need for additional lubricant and are incapable themselves of adding the additional lubricant necessary. Thus, operator attentiveness is a further requirement, especially in the presence of foam often occurring through churning of the lubricant due to the motion of operating elements therein.
As an alternative to purely manual systems, mechanical float systems were developed and applied to lubricant sumps, especially where multiple machines are in operation and the use of a central lubricant reservoir can be used to service the separate lubricant sumps of several machines simultaneously. Each of the lubricant sumps is provided with a float in fluid communication with the lubricant in the sump and mechanically linked with a float valve interspersed between the common reservoir and the individual lubricant sump. When the lubricant level in the sump drops below a predetermined level, the float mechanically linked with the valve opens the valve and allows lubricant to flow from the common reservoir to the lubricant sump in need of replenishing. Such a device is disclosed in U.S. Pat. No. 4,428,208 to Krause.
These mechanical float valves, however, are not very accurate and have a high failure rate due to compressor sump pressure pulsations and vibration. If failure of the float valve occurs and the compressor lubricant level drops, insufficient lubricant can result in "burn-out" due to main bearing scoring and/or input motor overheating.
Moreover, liquid refrigerant mixing with the gases in the sump and contributing to foaming of the lubricant in the sump can open the float valve and cause false feeding of lubricant into the sump. A high lubricant level of liquid refrigerant in the crankcase can cause the compressor to "slug" as lubricant is encouraged to enter the compressor compression chamber, which is designed to compress only a vapor. Excessive liquid pressurization caused by such a "slug" can damage a compressor's pistons, rings, connecting rods, crank shaft, or exhaust valves.
Moreover, such mechanical float valves typically have metal seats and valve needles, in an effort to improve durability in the presence of the compressor's pulsations and vibrations. Such needle valves are not "bubble tight" upon closure and can allow refrigerant to enter the compressor's sump through the lubricant feed and introduce lubricant-entrained refrigerant into the common discharge lines and to the common suction manifolds as described above. This can decrease system capacity. Moreover, transient lubricant level fluctuations tend to cause opening of the needle valve which can, in some instances, result in overfilling of the lubricant sump, creating the potential for hydrolocking the machine or "slugging" as noted above.
Optical sensors have also been recently developed for application to sensing the level of lubricant in lubricant sumps. An example is U.S. Pat. No. 5,103,648 to Barier. Therein, an optical sensor is used in conjunction with a sight glass to determine the level of the lubricant in the sump. When an insufficient lubricant level is determined, a solenoid valve in combination with the sensor is activated to allow additional lubricant into the sump and thereby replenish the quantity of lubricant in the sump to safe levels. However, it has been found with such optical systems that foaming of the lubricant can result in a false signal calling for the addition of lubricant when none may be needed. Further, the optical units have a high maintenance and failure rate and a low repeatability due to contaminants in a refrigeration system's working environment and compressor vibrations.
In additional to optical systems, capacitance and inductive sensors have been tried, but have been found unsatisfactory as they do not capably distinguish between lubricant, liquidified refrigerant, foaming lubricant, various types of lubricants and refrigerant, contaminated systems, or any of these combinations.